The Brazilian prodigy signed a long-term contract with the club after joining from Vasco da Gama during the January transfer window.
The youngster was immediately loaned back to his boyhood club on a short-term loan deal, while Chelsea tried to secure a work permit.
He only made six appearances for the Brazilian outfit but contributed a goal before returning to Stamford Bridge over the summer.
Santos was handed significant playing time during Chelsea’s pre-season tour and has made a lasting impression on new boss Mauricio Pochettino.
He was initially regarded as one of the youngsters who will remain with Pochettino’s squad this season, but the club’s transfer activity is expected to limit his first-team chances.
However, Santos remains in the club’s plans for the future, and they are keen to see the youngster continue his development at another English top-flight club.
Santos is expected to complete his medical soon, and Forest are trying to tie up the deal in time for the Brazilian to be available for their trip to Manchester United on Saturday.
Chelsea and Forest could be in contact again before the transfer window closes next week, with the Blues retaining an interest in Brennan Johnson.
The 22-year-old has emerged as a possible target for the Blues, who are looking to add another attacker to their ranks despite already splashing over £300m on new players this summer.
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